Detect a pixel with a specific color (OpenCV/C++)

answered 2015-03-08 14:03:46 -0600

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updated 2015-03-08 14:06:37 -0600

the function that you are looking for is the inRange(), for example:

Mat mask;
// specify the range of colours that you want to include, you can play with the borders here
cv::Scalar lowerb = cv::Scalar(255,127,39);
cv::Scalar upperb = cv::Scalar(255,127,39); 

cv::inRange(frame, lowerb, upperb, mask); // if the frame has any orange pixel, this will be painted in the mask as white

imshow("mask", mask); // show where orange pixels are located, then use this mask for further processing pass it for example to findNonZero() function in order to obtain the location of the pixels, etc...

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Thanks again Theodore! I'm using the follow code with image in example, but the output is just a black image. There's something wrong?

  Mat source = imread("chessOrange.png",1);
imshow("source", source);
Mat mask;
cv::Scalar lowerb = cv::Scalar(250, 125, 38);
cv::Scalar upperb = cv::Scalar(255, 129, 42);
cv::inRange(source, lowerb, upperb, mask);

imshow("mask", mask);

jefferson-sep ( 2015-03-08 18:00:00 -0600 )edit

yes your Scalar values are not correct. In opencv the formation is BGR and not RGB. So, your lower and upper limits should be Scalar(38, 125, 250) and Scalar(42, 129, 255) respectively.

theodore ( 2015-03-08 18:11:03 -0600 )edit

Oh, that explains why I could only detect white or black -- ' Thanks

jefferson-sep ( 2015-03-08 18:34:34 -0600 )edit

Let me comment that using inrange values on BGR image color channels is highly dependant on the avaible lighting conditions. It will miserably from the moment some sunlight or other lightsource comes into play :)

StevenPuttemans ( 2015-03-09 07:41:57 -0600 )edit

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Detect a pixel with a specific color (OpenCV/C++)